When an oil or gas well is no longer economical, or if there is a mechanical problem with the well which means that production is no longer possible or that well integrity has been compromised in some way, or for other reasons, the well may be abandoned. It is common practice to plug the well before abandoning it, e.g., to place barriers within the wellbore to prevent seepage of hydrocarbon product or water from the wellbore to the external environment. This can also apply to water injectors, i.e. bores which have been drilled in order to pump water into a reservoir to increase bottom hole pressure or to sweep hydrocarbon fluids to production wells.
Commonly, in a well with a good primary cement job external to the casing, plugging is accomplished by the placement of balanced cement plugs inside of the casing across from these well cemented intervals as required. The most common plugging medium is an oilfield cement slurry. The slurry can be designed to be gas tight and provide a barrier to upward fluid moment within the wellbore. When set and tested this creates a full cross-sectional barrier extending from inside the casing to outside of the casing and to the rock face.
Other plugging mediums such as settable plastic, barite and sized aggregate have also been used to place barriers internal to oilfield tubulars within well cemented external annuli intervals.
Cast iron bridge plugs, cement retainers and packers with a through-bore plug may be used as mechanical bases for placement of settable medium plugs.
Settable mediums can, by their nature, be very hard and exhibit brittle characteristics when subjected to external lateral force, such as compression or tension caused by subsidence or fault shifting of the strata in contact with the wellbore. If the settable medium is fractured post setting it possesses no mechanism to heal itself and regain cross-sectional integrity.
For plugging operations using cement, the plugging process often involves pumping a surfactant liquid, known as a “spacer”, down a drill string or work-over string (commonly referred to as the “string”) into the well taking returns up the string-by-casing annulus. The purpose of the spacer is to remove oil residues from the internal surface of the well casing and/or liner making them “water wet” (allowing better adhesion by cement). Commonly, immediately following the spacer, cement is pumped down the string and placed as a balanced plug. The string is then slowly removed from the wet balanced cement plug by pulling the string out of the hole until the bottom end of the pipe is above the cement slurry. The drill string and annulus are then circulated clean. The cement is allowed time to set and then the work string is used to tag (lowered down to contact resistance) and confirm the top of hard cement. Additional cement plugs can then be set with the same procedure or the string may be pulled from the wellbore.
If the plug is not correctly displaced or properly balanced prior to pulling the placement string above the cement slurry surface, the strength and integrity of the plug may be jeopardized because of cement contamination. The degree to which the cement plug integrity may be compromised can be difficult to assess.
Cement setting time can be monitored at surface by placing samples of the pumped cement slurry into a testing apparatus that recreates downhole conditions. When it has been determined that the downhole plug has achieved sufficient strength; the plug is pressure tested and tagged with the end of the work string to confirm the top of hard cement.
Abandonment plugs may also be placed in perforated completion intervals or in sections completed with screens.
To place a sealing plug inside of casing across from perforated intervals cement is most commonly used. To create an internal seal cement is squeezed through the perforations until the perforations are squeezed off and pressure builds within the tubular. This operation may require repeated squeeze operations before the perforations are squeezed (sealed) off and this repetition is time consuming. Incorporation of particulate material into the cement slurry along with ball sealers may also be required.
The sealing off of internal intervals with screens may be difficult because of the large open surface area. Cement has been used in conjunction with sized particulate material to seal off flow paths in order to fill the screen with cement and with the intent of retaining the cement inside of the screen. As flow paths are sealed off, the screen is progressively filled with cement plugging off the screen and creating an internal plug.